Generally, heavy machinery, such as excavators and forklifts, do not need to travel at high speed in order to serve their purpose. Therefore, it is sufficient for such heavy machinery to be shiftable between only two or three forward and reverse speeds so as to achieve its intended purpose.
As a representative technique related to the gear shifting system having three forward speeds and two reverse speeds, a transmission for heavy machinery, which is constructed such that it is directly coupled to a torque converter, connected to an output shaft of an engine to output power, and conducts gear shifting using a plurality of clutches and gear sets and transmits the power to a wheel shaft through a bevel gear unit, that is, such that the power is transmitted in the sequence of engine-transmission-bevel gear unit-differential gear unit-wheel shaft, was proposed in Korean Patent Laid-open Publication No. 2000-15284.
However, because the above-mentioned transmission is directly coupled to the torque converter without a reduction gear, gear shift shock in the transmission is excessive during a process of changing the speed of the engine output. Particularly, in order to shift the transmission from forward to reverse or from reverse to forward, the direction in which bevel gears of the bevel gear unit are rotated must be reversed. Therefore, there is a problem in that gear shift shock in the bevel gear unit is increased at the conversion point between the forward and reverse gears, thus increasing noise and deteriorating riding comfort.
In an effort to overcome the above problems, the inventor of the present invention proposed a transmission in Korean Patent Application No. 10-2004-2909, wherein the transmission is disposed between a bevel gear unit and a differential gear unit such that power transmission is conducted in a sequence of engine-bevel gear unit-transmission-differential gear unit-wheel shaft. In this technique, the power of an engine, which is first reduced in speed through the bevel gear unit, is input to the transmission. To shift the transmission to forward or reverse gear, hydraulic pressure is selectively applied to a forward clutch part or a reverse clutch part, thus controlling forward and reverse power transmission. Therefore, when the transmission is shifted to forward or reverse gear, the direction of the bevel gear unit can be maintained constant without changing. Thus, gear shift shock induced upon conversion between forward and reverse gear shifts is markedly reduced. In the transmission structure having such a power transmitting system, the bevel gear unit, the differential gear unit and the transmission are integrated into a single body, so that separate space for the transmission is not required, thus enhancing spatial freedom in designing the vehicle. In addition, because the power transmitting system is simplified, the weight of the vehicle and the number of elements are markedly reduced.
Furthermore, the inventor of the present invention proposed a compact transmission, which has a function of preventing heavy machinery from being momentarily moved backwards on an upward slope when gear shifting is conducted for forward movement, thus ensuring stability in work, in Korean Patent Application No. 10-2005-15918. The compact transmission according to this technique has a structure such that a subsidiary forward clutch part is additionally provided between the forward clutch part and the reverse clutch part of the transmission structure disclosed in Korean Patent Application No. 10-2004-2909. Thus, when the vehicle is in a stopped position or moves forwards, hydraulic pressure is supplied to a subsidiary forward clutch part to apply subsidiary forward power to the wheel shaft, thus preventing the vehicle from being moved backwards. As well, in the compact transmission, a one-way bearing is preferably provided in the subsidiary forward clutch part, such that only forward rotating power is transmitted to the wheel shaft, thus fundamentally preventing the vehicle from being moved backwards on the upward slope.
Moreover, the inventor of the present invention proposed, in Korean Patent Application No. 10-2005-26259, a multistage transmission, which is constructed such that, in the transmission structure having the power transmitting system of the engine-bevel gear unit-transmission-differential gear unit-wheel shaft, a separate planetary gear unit is provided on a power shaft connecting the transmission and the differential gear unit to each other, so that gear shifting including three forward speeds and two reverse speeds can be conducted, thus enhancing the operability of the vehicle. This technique will be regarded as the conventional art (hereinafter, referred to as ‘conventional multistage transmission’) and is explained in detail herein below.
FIG. 1 is a sectional view showing the construction of the conventional multistage transmission proposed by the inventor of the present invention.
As shown in the drawing, the conventional multistage transmission comprises a speed changing clutch part 2 and a planetary gear part 4. The speed changing clutch part 2 has a first clutch assembly 2a which is integrally coupled to a bevel ring gear 1 to receive power from the bevel ring gear 1, and a second clutch assembly 2b which is coupled to the first clutch assembly 2a and rotates integrally with the first clutch assembly 2a. The planetary gear part 4 has a first planetary gear assembly 4a, a second planetary gear assembly 4b and a reverse planetary gear assembly 4c. The planetary gear part 4 changes the speed of the power, transmitted from the speed changing clutch part 2, using the planetary gear sets.
The conventional multistage transmission further includes a stopping clutch part 3, a main drive gear 8a and a first carrier shaft 5c. The stopping clutch part 3 has a sun gear stopping clutch part 3a, a second carrier stopping clutch part 3b, a second ring gear stopping clutch part 3c and a reverse ring gear stopping clutch part 3d which respectively stop first, second and reverse sun gears 5a, 6a and 7a, a second carrier shaft 6d, a second ring gear 6c and a reverse carrier shaft 7d. The main drive gear 8a extends at a first end thereof inside the first clutch assembly 2a and is coupled at a second end thereof to a first ring gear 5c of the first planetary gear assembly 4a. The first carrier shaft 5c extends at a first end thereof inside the second clutch assembly 2b and is integrally coupled at a second end thereof to a rotating shaft of a first planetary gear 5b of the first planetary gear assembly 4a. 
Meanwhile, the differential gear unit 8, which is coupled to a rear end of the transmission, and a wheel drive unit 9, which is coupled to the differential gear unit 80, are sequentially coupled to the conventional multistage transmission, but these are not shown in the drawings.
However, in the conventional multistage transmission having the above-mentioned construction, the main drive shaft 8b must be relatively long, because the several sun gears 5a, 6a and 7a are provided on the main drive shaft 8b and gear shifting is conducted by the several planetary assemblies 4a, 4b and 4c, which respectively engage with the sun gears 5a, 6a and 7a. Particularly, because the input and output of power according to multistage gear shifting are implemented through the single main drive shaft 8b, there are problems in that the structural strength and stability of the transmission are reduced by work tolerance, and torsional instability results. To solve such problems, more precise machining is required. Therefore, the cost of manufacturing the transmission is increased.
Furthermore, in the conventional multistage transmission, multistage forward gear shifting and reverse gear shifting using the planetary gear assemblies is implemented using a construction such that the first carrier shaft and the second carrier shaft are used as an input shaft and the first ring gear is used as an output shaft. Accordingly, there is a problem in that the internal gear engagement makes it difficult to determine the reduction gear ratio.
In addition, in the conventional multistage transmission, due to structural and operational instabilities, there are technical limits in designing a transmission that has a function of preventing a vehicle from being momentarily moved backwards on an upward slope and also has the multistage forward and reverse gear shifting function, and in increasing the functionality thereof.
Therefore, it is required to develop a multistage transmission, which is provided between a bevel gear unit and a differential gear unit so that it has the advantages of the conventional multistage transmission, has superior structural strength and operational stability, facilitates determination of a reduction gear ratio, and has a function of preventing a vehicle from being momentarily moved backwards.